Merge lp://staging/~widelands-dev/widelands-website/move_maps_minimap into lp://staging/widelands-website

Move minimaps to wlmaps/minimaps
Implemented the new value needs_version_after
Delete map files (.wmf and .png) when deleting a map from the database

The maps minimaps are now moved to wlmaps/minimaps. This fixes also creating a faulty path in the database for the minimaps (beginning with a slash).

Deleting a map does now delete the related files.

The new value of 'needs_version_after' produced by wl_map_info is now considered. There is now an inline html file for this, so the logic is in one file.

The changes related to minimaps need manual intervention regarding the files and the database entries:

- All png files in MEDIA_ROOT/wlmaps/maps/ needs moving to MEDIA_ROOT/wlmaps/minimaps/
- Database entries for minimaps need to be changed from
  '/wlmaps/maps/NAME.png'
  to
  'wlmaps/minimaps/NAME.png'

To get this in:
- set maintenance
- backup the database
- backup the files from MEDIA_ROOT/wlmaps/
- run ./manage.py migrate
- move the png files
- correct the path(s) in the database. I want to change also some bad entries for the mapfiles. With activated virtualenvironment:

./manage.py shell
from wlmaps.models import Map
maps = Map.objects.all()
for m in maps:
    if m.minimap.name.startswith('/'):
        new_name = m.minimap.name.rpartition('/')[2]
        m.minimap = 'wlmaps/minimaps/{}'.format(new_name)
        m.save()

maps = Map.objects.all()
for m in maps:
    if m.file.name.startswith('/var/www'):
        new_name = m.file.name.rpartition('/')[2]
        m.file = 'wlmaps/maps/{}'.format(new_name)
        m.save()

This can maybe done also in one loop (changing minimap and file), but i fear saving an object twice in one loop can be problematic, although at home it works nicely.